1. Field of the Invention
The present invention relates to a fluid dynamic bearing apparatus, a spindle motor including the fluid dynamic bearing apparatus, and a disk drive apparatus including the spindle motor.
2. Description of the Related Art
In recent years, there has been a great demand for an increase in density as well as for a reduction in size, thickness, and weight, for apparatuses designed to drive recording disks, such as magnetic disks and optical discs, as used in personal computers, car navigation systems, and so on. Accordingly, there has been a demand for an increase in a rotation rate of spindle motors used therein to rotate the disks, and an improvement in accuracy of rotational operation thereof. In order to satisfy such demands, fluid dynamic bearing apparatuses, in which a gap between a shaft and a sleeve is filled with a lubricating oil, are now often used in place of traditional ball bearings, as bearing apparatuses for the spindle motors.
The fluid dynamic bearing apparatus includes a radial dynamic pressure bearing portion arranged to radially support the shaft or the sleeve, and a thrust dynamic pressure bearing portion arranged to axially support the shaft or the sleeve. Therefore, when the shaft and the sleeve are rotated relative to each other, dynamic pressure grooves provided in each of the radial dynamic pressure bearing portion and the thrust dynamic pressure bearing portion produce a pumping action to induce a fluid dynamic pressure on the lubricating oil filling a minute gap, thereby supporting the shaft or the sleeve radially and axially.
Spindle motors including such a fluid dynamic bearing apparatus are disclosed in JP-A 2002-5171 and JP-A 2005-48890, for example.
However, in conventional fluid dynamic bearing apparatuses, if the axial dimension of the shaft is reduced to make the fluid dynamic bearing apparatus thinner, the length of the radial dynamic pressure bearing portion is inevitably reduced to cause a reduction in radial stiffness. As a result, an external force, such as a shock, may cause a rotating member, such as the sleeve, or the shaft to tilt.
Also, in the case where a substantially cup-shaped member is adopted to maintain a sufficient length of the radial dynamic pressure bearing portion, the lubricating oil held in a minute gap between a lower surface of the rotating member including the sleeve and an upper surface of the substantially cup-shaped member, which is opposite to the lower surface of the rotating member, may come under negative pressure.
Hard disk apparatuses and optical disk apparatuses typically have spindle motors arranged to rotate disks installed therein. The spindle motor includes a stator portion fixed to a housing of an apparatus, and a rotor portion arranged to rotate while holding a disk. The spindle motor is arranged to generate a torque through magnetic flux generated between the stator portion and the rotor portion to rotate the rotor portion with respect to the stator portion.
The stator portion and the rotor portion of the spindle motor are arranged opposite to each other with a bearing apparatus intervening therebetween. In particular, in spindle motors in recent years, a fluid dynamic bearing apparatus using lubricating oil arranged between the stator portion and the rotor portion is often employed. JP-A 2010-121775, for example, describes a fluid dynamic bearing apparatus in which a gap defined between a stationary member and a rotating member is filled with lubricating oil as a bearing fluid.
Paragraph [0033] of JP-A 2010-121775 states that a radially extending gap and an axially extending seal gap continuous therewith are defined between a bearing sleeve and a second bearing component. In addition, paragraph [0033] also states that a pumping seal is defined in the seal gap.
Regarding such a fluid dynamic bearing apparatus, a vibration caused by, for example, transportation or the like of the fluid dynamic bearing apparatus may cause a change in the axial dimension of the radially extending gap. The change in the axial dimension of the radially extending gap may cause an air bubble to be generated in a portion of the bearing fluid which is held in the radially extending gap due to cavitation. Moreover, discharge of the air bubble to an outside is prevented by the pumping seal, so that the air bubble may reside inside of the pumping seal. This air bubble may lift a gas-liquid interface of the bearing fluid to such an extent as to cause a leakage of the bearing fluid.